Porous pavement for water quantity and quality management

ABSTRACT

A pavement material for the capture of waterborne constituents. The pavement material comprises a porous pavement substrate and an amphoteric compound bonded to the substrate. The porous pavement material may have a hydraulic conductivity ranging from about 0.001 to about 1.0 cm/sec and may act as a storm water storage basin.

This is a divisional of Ser. No. 09/714,366, filed on Nov. 16, 2000, towhich this application claims priority and which is hereby incorporatedby reference in its entirety.

BACKGROUND OF INVENTION

The present invention relates to the removal of waterborne metalcontaminants from water. In particular, the invention relates to afilter media which removes metal contaminants from water passed throughthe filter media.

An area of increasing concern in the environmental sciences andengineering is the treatment of metals such as Cd, Cu, Zn, Ni, Pb, andCr, which become waterborne and are carried by rain water run-off andthe like to environmentally sensitive areas. As used herein, metalsbeing “waterborne” means being transported by water in any manner,whether the metal is actually in solution, suspended in water through aparticulate bond or a colloidal bond, or simply physically carried bythe velocity of flowing water. One of the most common manners in whichmetals become waterborne is through entrainment with storm water run offfrom road surfaces. The above metals are typically deposited on the roadsurface though vehicle exhaust, fluid leakage, vehicular wear, pavementdegradation and pavement maintenance. Subsequent rainfall entrains themetals and transports the metals to the area in which storm waterrun-off accumulates. Typically, 60% to 80% of these metals are dissolvedin the run-off water, while the remaining percentage is suspended byother mechanisms such as those mentioned above.

It is desirable to intercept the runoff and remove the metals prior toallowing the water to continue to its natural drainage areas. One methodof removing the waterborne metals is to pass the water through a sorbentfilter media. One of the most common media for removing particulatebound metals from water is sand. However sand has very little capacityfor removal of dissolved metals and therefore, is generally notconsidered effective in removing dissolved metals. Granular activatedcarbon (GAC) has long used as a media for removing dissolved metals.However, GAC has relatively little absorptive capacity and thus,absorbed metals must frequently be removed or the GAC “recharged.” Also,GAC has very little compressive strength. Any application which places aload on the GAC material may cause crushing and a greatly reduceabsorptive capacity of the GAC.

A much more recently developed sorbent media is iron oxide coated sand(IOCS). IOCS is formed by coating silica sand with a thin layer of ironoxide and it has been shown to be an effective sorbent media for metals.Iron oxides and hydroxides possess little or no permanent surfacecharge, but will take on a positive or negative surface charge in thepresence of protons or hydroxyl ions. In other words, depending on thepH of the solution in which the iron oxide is place, the iron oxide maytake on a net positive or negative charge. A substance which exhibits anet positive or negative charge depending on the pH level may bereferred to as an “amphoteric” substance.

Iron oxide typically has a neutral charge in a pH range of approximately7 to 8. When the pH rises above approximately 8, the iron oxide becomesmore negatively charged. Thus, positively charged metal ions borne bywater passing over the negatively charged iron oxide will tend to bondto the iron oxide and be sorbed from the water. Conversely, if the pHfalls below approximately 7, the iron oxide becomes positively chargedand is less likely to bond with metal ions. The pH at which the netsurface charge of a particle is zero is denominated the point of zerocharge or “pzc”.

One major disadvantage of IOCS is that the oxide coating is notsufficiently durable. The comparatively smooth surface of sand particlestends to result in the oxide coating flaking off. Attempts to avoid thisflaking have led to time consuming sand preparation efforts such ascleaning the sand of organics and applying a scratch surface to the sandbefore applying the oxide coating. However, even with these preparationefforts, IOCS still exhibits considerable flaking and thus a lack ofoxide coating durability.

The smooth surface of sand is also disadvantageous from the standpointof providing a comparatively low specific surface area (SSA). Thespecific surface area of a material is generally defined as the surfacearea per unit mass with the typical unit being m²/gm. As used herein,specific surface area means the total area on the surface of thematerial in addition to any available porous internal surface area (suchas found the GAC discussed above). The greater the surface area of thesubstrate, the greater the surface area of oxide coating which will beexposed to waterborne metals. Thus, it is desirable to provide asubstrate with as great of an SSA as possible considering other designrestraints. The SSA of sand is typically about 0.05 to about 0.10 m²/gm.

Another problem found in IOCS is the tendency of the oxide coating tocrystallize. When the coating crystallizes, the crystals set up auniform lattice which does not maximize the surface area of the coating.The surface area of the coating is much more optimal if the oxidemolecules are randomly distributed in a non-lattice or “amorphous”fashion. For example, the SSA of IOCS may reach 85 m²/gm if a method ofsufficiently inhibiting crystallization could be provided. However, apurely crystallized oxide coating may have a SSA as low as 5 m²/gm. Whatis needed in the art is a manner to reliably inhibit crystallization inIOCS. Even more desirable would be a substrate other than sand which hasa higher SSA than sand and a superior tendency to inhibitcrystallization. It would also be desirable to provide substrates whichcould simultaneously act as a filter and provide other functions, suchas providing a roadway pavement or parking pavement. Another desirablecharacteristic of a substrate (such as porous concrete) would beproviding pH elevation to the fluid stream being treated.

SUMMARY OF THE INVENTION

One embodiment of the present invention is an adsorptive-filtrationmedia for the capture of waterborne or airborne constituents. The mediacomprising a granular substrate and an amphoteric compound bonded to thesubstrate in the presence of a crystal inhibiting agent.

Another embodiment of the present invention includes anadsorptive-filtration media which comprises a substrate having aspecific gravity of less than 1.0 and an amphoteric compound bonded tothe substrate.

Another embodiment is a pavement material for the capture of waterborneconstituents. The pavement material comprises a porous pavementsubstrate and an amphoteric compound bonded to the substrate.

Another embodiment includes a process for producing anadsorptive-filtration media for the capture of waterborne or airborneconstituents. The process comprises the steps of providing a substratewith a specific surface area of greater than 0.1 m²/gm, introducing thesubstrate to an amphoteric metal solution, and drying the substrate.

Another embodiment includes an adsorptive-filtration media whichcomprises a substrate with a specific surface area of greater than 0.1m²/gm and an amphoteric compound bonded to the substrate.

A further embodiment includes a storm water storage basin capable ofsupporting vehicular traffic. The basin comprise a layer of porouspavement having a hydraulic conductivity of more than 0.0001 cm/sec. Thelayer of porous pavement is at least 3 inches in depth, and the layerhas a length and a width wherein the ratio between the length and thewidth is less than 20.

Another embodiment includes a method for producing a porous,cementitious material. The method includes the steps of providing andthoroughly mixing cement and aggregate, mixing water with the cement andaggregate into a slurry while maintaining a water to cement ratio ofless than one, initiating curing of said slurry under pressure and inthe presence of steam, and continuing the curing at ambient temperatureand pressure until the cementitious material is substantially dry.

Another embodiment is a roadway with a gravel shoulder for the removalof waterborne ionic constituents. The roadway comprises a pavementsection and a gravel shoulder section adjacent the pavement section. Thegravel shoulder has a depth of at least 3 inches and includes gravelcoated with an amphoteric compound.

Another embodiment includes a method of constructing a sub-base for theremoval of waterborne constituents. The method includes the steps ofplacing a layer of uncompacted sub-base material; distributing upon thelayer a solution containing an amphoteric compound; and compacting thelayer to a selected density.

Another embodiment is an adsorptive-filtration media for the capture ofwaterborne or airborne constituents. The media comprises a flexible,planar, porous substrate; and an amphoteric compound bonded to saidsubstrate.

Another embodiment is a drainage pipe capable of capturing waterborneconstituents. The drainage pipe comprise a length of pipe having aninterior surface, at least a portion of the surface being designed to bein contact with water. An amphoteric compound is then applied to theportion of the surface designed to be in contact with water.

Another embodiment of the invention includes a process for creating afiltering media for the capture of waterborne or airborne constituents.The process comprises the steps of providing a filter substrate;applying a first coating of an iron oxide compound to the substrate; andapplying a second coating of a manganese oxide compound to thesubstrate.

Another embodiment of the invention includes a roadway with a shoulderforming a filter for constituents. The roadway comprises a roadwaypavement section and a cementitious, porous, shoulder adjacent thepavement section and the shoulder having an amphoteric compound appliedthereto.

Another embodiment provides an absorptive-filtration media have a porousstructure of a fixed matrix and a porosity of approximately 0.05 to 0.6and an amphoteric.

Another embodiment provides an adsorptive-filtration media having agranular substrate and an amphoteric compound formed of a manganeseoxide formed on the substrate.

A another embodiment includes a method for forming a porous pavementroadway. This method includes the steps of providing and thoroughlymixing cement and aggregate; mixing water with the cement and aggregateforming it into a slurry while maintaining a water to cement ratio ofless than one; and placing the slurry into a roadway bed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of an upflow filter.

FIG. 2 is a chart of surface charge versus pH for certain amphotericcompounds.

FIG. 3 is a chart of aggregate distribution.

FIG. 4 is a cross-section of a roadway.

FIG. 5 is a conceptual representation of the pavement storage basin ofthe present invention.

DETAILED DESCRIPTION

The filtering media of the present invention generally comprises anadsorptive-filtration media. The media is adsorptive in that it willinclude a substrate with an amphoteric compound bonded to the substrateto produce a high specific surface area media capable of adsorbing (i.e.the physico-chemical capture of) ion contaminants. The media may alsohave a filtration characteristic which relies on the purely physicalcapture of contaminants which are larger than the void spaces in themedia.

In one general embodiment, the invention includes a granular substratewith an amphoteric compound bonded thereto in the presence of a crystalinhibiting agent. The granular substrate could be sand or any othergranular substrate such as crushed limestone, crushed concrete, or othergranular substances. In another general embodiment, the presentinvention includes all substrates having a specific surface area of 0.1m²/gm or greater and having an amphoteric compound bonded to thesubstrate. In these latter embodiments with a substrate having an SSA ofgreater than 0.1 m²/gm, the substrate could include a wide variety ofmaterials such as precast cementitious porous pavement (CPP) discussedherein (SSA of 5-10 m²/gm), wood chips, recycled concrete chips,recycled concrete pavement rubble, natural aggregates, syntheticaggregates, polymeric compounds, granular activated carbon (SSA of600-1200 m²/gm) etc.

1. Iron Oxide Coated Media

The amphoteric compound of the present invention is intended to includeany compound having amphoteric properties. Preferred embodiments ofamphoteric compounds include oxides of iron, manganese, or aluminum.

In regards to iron oxide compounds, there are at least 13 iron oxideminerals, of which there are 8 major iron oxides. These iron oxidesdiffer in composition, the valence state of Fe and in crystallinestructure. However all iron oxides contain Fe and O or OH. Table 1summarizes the major iron oxides with selected characteristics.

TABLE 1 Selected properties and attributes of major iron oxide minerals.Mineral Structural Density SSA Name Formula system (g/cm³) (m²/g) ColorHematite α-Fe₂O₃ Trigonal 5.26 20-30 blood red Maghemite γ-Fe₂O₃ Cubicor tetragonal 4.87 80-130 chocolate Magnetite Fe₃O₄ Cubic 5.18 ˜4 blackGoethite α-FeOOH Orthorhombic 4.26 20-40 mustard Lepidocrocite γ-FeOOHOrthorhombic 4.09 70-80 orange- brown Ferrihydrite¹ 5Fe₂O₃9H₂O² Trigonal3.96 180-300 deep brown Feroxyhyte δ'-FeOOH Hexagonal 4.20 190-210 brownAkaganeite β-FeOOH Tetragonal 3.56 ˜30 dark mustard ¹: ferrihydrite &feroxyhyte have the only amorphous or poorly-crystalline structures lowSSA from Fe(NO₃)₃9H₂O hydrolysis, high SSA from Fe³⁺ precipitation withKOH ²: other formulas include: Fe₅HO₈4H₂O and Fe₆(O₄H₃)₃ point of zerocharge (pzc) for all minerals shown is between pH 7-8 α: hexagonal closepacked (more stable than γ) β: goethite polymorph in presence of highCl⁻ levels γ: cubic close packed δ': poorly-ordered ferromagnetic formof FeOOH

From Table 1 it can be seen that the more amorphous ferrihydrite orferoxyhyte are the forms of iron oxide with the highest SSA. If theseforms are coated onto silica sand, their higher SSA, as compared to saythe more crystalline hematite, will create a more preferable sorbentmedia. For this reason, a preferred embodiment of the amphotericcompound focuses on the use of these forms, specifically ferrihydrite.Those skilled in the art will understand that ferrihydrite is notproduced in isolation, but is typically formed in a solution havingvarious other iron oxide compounds. The ferrihydrite may transform intoother, more crystalline iron oxide compounds (such as hematite orgoethite) depending on factors such as temperature, pH, and whether theiron source is ferric or ferrous ions. To inhibit such transformation tothe more crystalline compounds, inhibiting agents such as silica (SiO₂),silica fume or silica gel, inorganic compounds such as phosphates,polymeric compounds whether naturally occurring (e.g. natural organicmatter in soil) or synthetic (e.g. polyethylene), sodium hydroxide,oils, grease, or any other substance which inhibits crystallization, maybe introduced in the process for synthesizing ferrihydrite or applyingthe iron oxide coating. Where sand is the substrate, a highly acidiccompound, such as ferric nitrate or ferric chloride (used to form theamphoteric compound as described below) may soluablize silica off thesand substrate, thereby producing an inhibiting agent. Because so manysubstances may act as inhibiting agents, it is possible that certainimpurities in the materials selected (such as grease or oil in a sandsubstrate) can be engineered to act as a sufficient inhibiting agentwithout the addition of further inhibiting agents.

Two known methods for producing ferrihydrite follow. The first methodinvolves preheating 2000 mL of DI water to 75° C. in an oven and thenwithdrawing the water and adding 20 g of unhydrolyzed crystals ofFe(NO₃)₃ 9H₂O. The solution is stirred rapidly and reheated at 75° C.for 10 to 12 minutes. The formation of iron hydroxy polymers will changethe solution from a dull gold color to dark reddish brown. The solutionis then dialyzed for three days to produce approximately 5 g offerrihydrite. This procedure produces a ferrihydrite of lower SSA, inthe range of 180 to 200 m²/g.

A second method involves dissolution of 40 g of Fe(NO₃)₃ 9H₂O in 500 mLof DI water and addition of approximately 330 mL of 1 M KOH until the pHis 7 to 8 while stirring the solution. This procedure produces aferrihydrite of higher SSA, in the range of 200 to 300 m²/g. Thesolution is then centrifuged and dialyzed to produce approximately 10 gof ferrihydrite. While both of these procedures work well for a smallmass of ferrihydrite (i.e. 10 g) in a laboratory environment, they arenot easily adapted to be economically feasible at production or fieldscale levels that require tons of such a coating. Rather, the abovemethods would require design and construction of a plant-sized processto produce multiple tons of ferrihydrite.

The present invention includes another, more economical method forproducing sufficient quantities of ferrihydrite. In this method, thesource of ferric ions is either Fe(NO₃)₃ 9H₂O, (ferric nitrate (FN)) orFeCl₃, (ferric chloride (FC)). Both FN and FC are available asreagent-grade salts or available commercially in larger quantities asbulk solutions. FC has the additional advantage of being more economicaland being a by-product of pickling waste. When FN or FC are dissolved inpotable water to produce an approximately 1 M to approximately 3 Msolution, the resulting iron oxides in the solution will typically beapproximately 50% ferrihydrite and 50% other iron oxides.

2. Applying Iron Oxide to a Substrate

One substrate to which may be coated with the amphoteric compound may beadhered is sand. Sand typically has a comparatively low SSA of about0.05 to about 0.10 m²/gm. Moreover, this low SSA is indicative of arelatively smooth surface to which iron oxide coatings will havedifficulty adhering. As discussed above, without some agent to inhibitcrystallization of the iron oxide coating, the SSA may remain in therange of 1 to 5 m²/gm. To produce a sand substrate filtration media witha markedly improved SSA (about 5-20 m²/gm), sand was subject to amulti-step process as seen in the following two examples.

In the first example, the sand was first cleaned and tumbled in acidicsolution (of a pH<2), rinsed with DI water, and then cleaned and tumbledin a very dilute basic solution before a final rinse is made. Second, topromote bonding, an initial scratch coat applied by immersing the sandin an approximately 1 M FN solution. The sand was then heat at about 100degrees C. until this coating was dry and then the sand wasdisaggregated and rinsed in DI water to remove any loose coating. Afterthis rinsing, the sand was reheated until dry and then cooled. Third,the sand was immersed in another solution of 1.6 M FN. In this solution,1,000 ppm SiO₂ was added (in the range of 1% of the aqueous volume) tohelp inhibit the transformation of ferrihydrite to hematite or possiblyto goethite. Fourth, the sand was again dried with drying timesminimized in order not to promote the transformation to hematite due todehydration. However, drying of the sand at high temperatures could alsolead to thermal transformation of ferrihydrite to hematite. It wasdetermined that drying could take place at an acceptably fast rate at100° C. if an inhibitor such as SiO₂ was used to prevent crystallinebonds from forming. Once drying was complete, the sand was allowed tocool and the coated media was disaggregated. As a final step, the mediawas pH conditioned to a neutral pH by passing DI water at a pH of 8 to 9(raised with NaOH or a similar base) through the media until the pH ofthe effluent was between 7.5 and 8, above the point of zero charge foriron oxides. This also removed any loose iron coating. It is noted thatthe above mentioned scratch coating is necessary because the granularsubstrate was sand which has a relatively smooth surface. However, othergranular substrates such as crushed limestone have a sufficiently roughsurface that a scratch coat is not required.

The second example is provided by a large-scale field production. Theabove method is scaled up by using a larger gasoline-powered concretemixer and a gas-fired heater. A 3.0 M ferric chloride (FC) solutioncontaining 1000 ppm silica solution was prepared in sufficient volumesuch that the sand could be completely immersed. Thereafter, heat wasapplied via the gas-fired heater to evaporate the liquid and attach theiron to the sand surface. Typically greater efforts must be made toinsure dryness of the FC treated sand as opposed to the FN treated sandsince FC is significantly more hydroscopic than FN. This method provedfeasible to produce the required 9 tons of OCS necessary for a relatedexperiment.

For each batch, approximately 90 pounds of filter sand was placed in theconcrete mixer with an excess of ferric chloride solution. The amount offerric chloride solution put into the mixture was enough to just coverthe filter sand. The mixture was stirred vigorously and heat applied bythe a gas-fired heater. The gas-fired heater was directed into the mouthof the concrete mixer. The slurry was continuously stirred by theconcrete mixture until the sand was completely dry. Typical drying timefor each batch was 3 hours.

Once dry, the sand was poured from the concrete mixer into a backhoebucket and placed in a tandem dump truck for cooling. In preparation forpH neutralization, complete drying of the sand was essential to ensurethe iron coating would not be removed by the sodium hydroxide in the pHneutralization process. If the sand is not completely dry, the ironcoating washes off easily when put into the NaOH solution.

Since the sand was placed in a tandem dump truck for cooling, it decidedto neutralize the entire truckload at once to reduced handling of theOCS. The dump truck full of OCS was parked facing down a slope and asolution (of approximately 10 lbs. of NaOH per 55 gallons of potablewater) was poured into the truck bed on top of the OCS. The idea was tocreate a bathtub effect to neutralize the sand. The truck bed did leakbut the level of the solution was kept above the depth of the sand withcontinual addition of NaOH solution. Leakage of the truck bed provedbeneficial due to the continual addition of new solution to replaceloss. The new solution was more capable of neutralizing the OCS whilethe used solution was removed from the system. The pH was checked with apH probe at several depths in the truck bed to ensure completeneutralization. Approximately 10 tons of OCS was produced, the largestknown quantity of such material. In the above process, the inhibitingagents were formed by the impurities found in the mixer, the gas-firedheater, NaOH and the construction process in the field to such a degreethat it was not necessary to add additional silica as an inhibitingagent.

3. Substrates with a Specific Gravity Less than 1.0

There are a large number of likely substrates having an specific gravityof less than 1.0. One family of such substrates is wood, with pinehaving by way of example a specific gravity of about 0.35. Anotherfamily of such substrates are polymeric compounds. Polymeric compoundsmay include light weight materials such as foam packing pellets, whichwould form a granular media having a specific gravity of approximately0.2. Polymeric compounds could also include heavier polymers having aspecific gravity of up to 0.97. Polymeric compounds could also includepolymer-type materials which have similar weight, flexibility, and longmolecular chains. Of the polymer family, it has been found thatpolyethylene (PE) or polypropylene (PP) have many characteristics makingthem suitable substrates for the present invention. PE and PP have aspecific gravity of about 0.9. It is believed PE, PP, and other similarpolymeric compounds are particularly useful when in the form ofpolymeric floating media filter beads. Normally, polymer beads will havea specific gravity ranging between approximately 0.50 and 0.95. Onesimple example of a “filter” or “clarifier” using floating polyethylenebeads can be seen in FIG. 1. In the embodiment of FIG. 1, the filter isa cylindrical geometry upflow filter, but the filter could utilize manygeometries and flow directions depending on constraints such as mediatype, coating, specific gravity and design intentions. Filters usingfloating polyethylene beads are usually upflow filters such as seen inFIG. 1, but can be downflow filters and have a variety of geometricshapes. In FIG. 1, the upflow filter 10 is filled with floatingpolymeric beads 12. An influent flow 13 flows into filter 10, throughbeads 12 (where it has pollution constituents adsorbed and filtered),and exits as effluent 14. While not explicitly shown in FIG. 1, theupflow filter 10 could utilize any number of methods well known in theart for backwashing the beads. Upflow filters have the advantages ofbeing easily backwashed to prevent clogging and are less likely tohydraulically “short-circuited” (i.e. water cutting an uninterruptedfluid path through the beads and not having to flow around theindividual beads). It has been found that allowing a layer of sedimentto form at the base of the filter media may actually enhance filtrationas long as the layer does not become so thick that the layersignificantly inhibits design flows. The filter media would bebackwashed at the point design flows were significantly inhibited. It isalso very practical to direct water through an upflow or downflow filterwhen the water is being drained from a elevated grade (such as a highwayoverpass or an elevated interstate).

One preferred method of applying the amphoteric compound to thepolyethylene is similar to that used to apply iron oxide to sand and isas follows. A 1 to 3 molar solution of FN or FC (preferably about 1.6 M)is prepared by dissolving the FC or FN in water. The polyethylene beadsare placed in the solution and continuously stirred. The polyethyleneshould remain in the solution a sufficient time for the entire surfacearea of the polyethylene to become coated with iron oxide. An hourshould be sufficient period of time under most circumstances. The wateris then evaporated from the solution containing the polyethylene at atemperature of approximately 90°-95° C. The drying may take place atlower temperatures, but will unnecessarily slow the drying process.Drying at higher temperatures is possible, but may be undesirable fromthe standpoint of the polyethylene becoming excessively plastic attemperatures above 95° C. and crystallization of the iron oxide becomingmore prevalent at higher temperatures.

One favorable characteristic of employing polyethylene as a substrate isthat polyethylene has an inherent tendency to inhibit thecrystallization of the iron oxide. This is believed to occur by way ofpolyethylene molecules detaching from the substrate surface and becominglodged in the iron oxide molecules depositing on the substrate surface.As alluded to above, this disruption of a uniform iron oxide latticetends to create a favorable, amorphous (thus high specific surface area)coating of iron oxide. In addition to taking advantage of the naturalcrystallization inhibiting character of polyethylene, when using an ironoxide as the amphoteric compound, it also desirable to further add aninhibitor such as the 1000 ppm SiO₂ solution discussed above. The amountof SiO₂ solution may vary, but an amount equal to 1% of the aqueousvolume is normally considered sufficient. If manganese oxide is theamphoteric compound, it usually is not necessary to add an inhibitingagent to achieve an acceptable SSA. Significantly, it has been foundthat polyethylene beads having a specific gravity of about 0.9 maintaina specific gravity of less than 1 (and therefore float) even after beingcoated. The coating generally raises the bead's specific gravity toabout 0.95.

While the above procedure described applying an amphoteric compound topolyethylene beads, it will be understood that the procedure could becarried out numerous other polymeric materials. For example, anamphoteric compound could be applied to simple packing material, cheappolymeric woven and non-woven material, geosynthetics and expanded foamsas well. The foams have to be dried at a lower temperature so they donot melt, so for the case of expanded foams or heat sensitivepolymerics, manganese coatings are preferable to iron coatings (whichrequire higher temperatures to dry).

4. Manganese Oxide Coated Media

As mentioned above, another family of amphoteric compounds are oxidesformed from manganese. There are a whole series of manganese oxideminerals that can be produced that have useful characteristics as mediacoatings for the treatment of storm water and other waste streamscontaining heavy metals. However, two manganese oxides groups comprisepreferred embodiments for use with the present invention because theircombination of negative surface charge (measured as units of charge persurface area) at nearly all environmental pH values and because of theirhigh specific surface area. This results in a coated media surface witha high surface density of negatively charged sites for adsorption ofheavy metals. These two manganese oxides are birnessite (whose structureis not completely understood, but is believed to be in part a layered(MnO₆) structure and cryptomelane, (α-MnO₂) which is a tunnel structure.Both are different manganese oxide minerals having different structures.Although not as critical as with iron oxides, some inhibition ofcrystallization may be helpful to produce poorly crystalline structuresand higher surface area

The point of zero charge (pzc) of manganese oxides and the surfacecharge density are the keys to an important advantage of manganese oxidecoatings over iron oxide coatings in the adsorption of heavy metals.Iron oxide coatings only have a negative charge on their surface whenthe pH of the solution surrounding the media is greater than the pzc ofthe coating. For pure iron oxides crystalline minerals, this ranges from7 to 8 depending on the mineral form of iron oxide (i.e. goethite,hematite, etc.) and is a comparatively narrow range. Forsilica-inhibited ferrihydrite this pzc can be between pH values of 5.5to 7.5. For manganese oxides the pzc values are much lower. The pzcoccurs at a pH of less than 5. Reported values are in the range of 2 to3. FIG. 2 illustrates the pzc for the manganese oxides Birnessite andCryptomelane and the iron oxide Goethite. Thus, for manganese oxidecoated media there is a strong negative charge at typical environmentalpH levels of 6 to 8. This also means that pH conditioning such asrinsing with DI water is usually not necessary for manganese oxidecoated substrates.

Those skilled in the art will recognize there are numerous methods ofproducing manganese oxides for use in the present invention. Thefollowing two methods disclose one preferred method of the presentinvention for producing both birnessite and cryptomelane.

Method A; Birnessite Coating Method (BCM)

The disclosed birnessite coating method uses a wet oxidation procedureto precipitate the colloid of birnessite on the media surface. In otherwords, a solution containing manganese was oxidized to create an MnO_(x)form. Two moles of concentrated hydrochloric acid (37.5%) were addeddropwise and continuously to a boiling solution of 0.5-M potassiumpermanganate in 1 liter of water, to which 0.5 liters of media wasadded, immersed and vigorously stirred. The media actually used includedplastic beads, sand, GAC, concrete blocks and concrete rubble. However,any other suitable media (wood, etc.) could also be used. After boilingfor further 10 minutes, the media was washed with water and dried atroom temperature overnight. Under lab conditions, a reasonably pure formof birnessite can be produced (>80 pure). This produced a coating havinga surface area of 70-90 m²/g (i.e. surface area of coat as applied tothe substrate) with a pzc at a pH near 3. At environmental pH values thesurface charge density is very negative (−10 to −20 micromoles/m²). Thiscoating has an approximate mean of about 1200 micromoles of negativecharge per gram of coating.

Method B; Cryptomelane Coating Method (CCM)

The Cryptomelane coating method uses a wet oxidation procedure toprecipitate the colloid of cryptomelane on the media surface. A solutionof 0.35 moles KMnO₄ in 800 ml of water is heated to 60° C. and dropwisecontinuously added into a solution 0.5 moles of MnSO₄ in one liter of 2M acetic acid. This solution was heated with 500 ml filtration media(such as acid washed polyethylene beads or any of the media types namedabove) to 80° C. while vigorously stirring. After stirring for 15minutes, the media was removed, filtered, washed with water and allowedto dry at room temperature overnight. Under lab conditions a reasonablypure form of cryptomelane can be produced (>80 pure). This will producea coating having a surface area of 200 to 270 m²/g (i.e. the surfacearea of the coating itself rather than applied to the substrate asabove) with a pzc at a pH near 3 to 4. At environmental pH values thesurface charge density is very negative (−2 to −5 micromoles/m²). Thiscoating has an approximate mean of about 823 micromoles of negativecharge per gram of coating.

It will be understood that the most significant factor is thecombination of specific surface area and surface charge. The differencebetween 1200 and 823 can be important when these coatings are appliedconsistently as with a chemical process operation. It should be notedthat at the upper end of environmental pH values, ferrihydrite (ironoxide) has a surface area of between 200 and 300 m²/g and a surfacecharge density of −0.1 to −1.0 micromoles/m². Silicate (a form ofsilica) contamination (addition of silica solution or natural silica inclay minerals), tends to prevent ferrihydrite from transforming to otheriron oxides and thus tends to keep the pzc at a pH of around 5.5 to 7.5,as is typical for ferrihydrite. This coating has an approximate mean ofabout 113 micromoles of negative charge per gram of coating. However,the cost of an iron oxide coating is approximately {fraction (1/10)} to⅕ of a manganese coating. This cost does not include the cost of pHconditioning of the influent for iron oxides which can be significantfor engineered systems.

Those skilled in the art will recognize that there is a variety ofsynthetic manganese oxide minerals as there is with iron oxide minerals.However, manganese oxides have not been as well studied as iron oxides.Technically, the term “birnessite” is used to refer to a group ofmanganese oxides for which the exact structures are still to a certainextent unknown. What is known is that these birnessite minerals arelayered structures. Examples of birnessite minerals having a valence >+4are vernadite, ranciete, buserite, and lithiophorite. Examples ofbirnessite minerals with a valence <+4 are magnetite and hausmannite.The other manganese oxides are tunnel structures. One of the more commonis cryptomelane which forms a group of manganese oxides along withhollandite and coronadite (all having α-MnO₂ structures with a largeforeign cation (K, Ba or Pb respectively) as part of the structure).Other minerals include ramsdellite (β-MnO₂),Nsutite (ρ-MnO₂),romanechite (MnO₆) and todorokite. All of these minerals have negativesurface charges and have SSA's that fall in the range of 50 to 280 m²/g.Birnessite and cryptomelane are easy to produce and provide a goodcombination of negative surface charge and SSA for adsorption ofcationic species (mainly heavy metals) when the pH is above the pzc (seeFIG. 2). Naturally, it will be understood that lowering the pH below thepzc will allow the removal of anionic species such as nitrite (NO₂ ⁻),nitrate (NO₃ ⁻), or phosphates (PO₄ ⁻).

It will be recognized the choice between iron oxide and manganese oxidepresent a typical design choice which will be governed by the particularengineering problem being addressed. Additionally, differentconcentrations of the metal oxides have been used in the solutions inwhich the substrate is immersed. The concentrations may range from 0.1 Mto 3.0 M (or higher) solutions of the metal oxide. Nor is the inventionlimited to immersing the substrate in a metal oxide solution. Rather,the metal oxide solution could be an aerosol which is spayed onto thesubstrate. This technique works well in a reactor that fluidizes themedia using a gas such as air. The metal oxide coating is injected as afine spray onto the fluidized media. Once the media is coated, thetemperature in the reactor would be raised to evaporate off the waterand leave the oxide coating on the media. The media will continue to befluidized throughout this process. The reactor can be as simple as anupflow column or a conical upflow reactor. A significant advantage ofthis technique is the savings created by the efficient use of thecoating material.

Although not as generally preferred as iron oxides or manganese oxides,aluminum oxides may also be a viable oxide coating, especially onmaterials such as CPP. The chemistry of aluminum oxide indicates that itshould be a viable material and the cost of this material is relativelylow. Therefore, aluminum oxides (such as forms of Al₂O₃) used asamphoteric compounds are intended to come within the scope of thepresent invention.

The advantage of various alternative embodiments of the presentinvention will become apparent as those skilled in the art begin topractice the invention. For example, using cementitious porous pavement(CPP, discussed below) as the filter media or coating substrate allows aunique manner of avoiding the cost of pH conditioning of the influent.As is well known, cement is largely composed of alkalinity-producingsubstances and therefore is capable of pH elevation. One method is tocoat only the bottom half of a CPP pavement block with iron oxide ormanganese oxide. Then, as pavement runoff percolates down through theupper exposed cementitious material near the pavement surface, the pH ofthe percolating runoff will be elevated above the pzc of the iron oxidecoating on the lower half of the CPP block and thus, the lower half ofthe CPP block form an efficient passive fixed adsorption matrix.

5. Cementitious Porous Pavement (CPP)

Those skilled in the art will recognize many design issues which applyto the choice of substrates or filter media. As discussed above, themedia may be many materials such as sand, polyethylene beads, or a fixedporous matrix such as cementitious porous pavement (CPP). Typically, theprior art is only concerned with making cement structures as imperviousto water as possible. However, one aspect of the present invention iscreating a cement substrate which is quite porous. A wide range of sizeand gradation of material may be used as media and CPP blocks may beused in their block form or broken up to serve as a rubble media. Issuessuch as contact time, contact surface area, filtration ability, andhydraulic conductivity required will determine the choice of media orrubble size. Any of the above described amphoteric coating preparationtechniques may be applied to CPP material either as the material isbeing produced (described below) or after the material has been producedwithout a coating (in large or small blocks or as sections). If the CPPmaterial is not produced with the amphoteric compound as an admixture,the block of material will be immersed in the amphoteric coatingsolution of choice and the solution is circulated through and around theCPP block. Because of contact time issues, one preferred method requiresthe intact CPP blocks to remain in the circulating manganese oxidesolution for 60 minutes before removing and drying. Drying may takeplace at room temperature for several days under still air conditions orfor 24 hours when air is being blown by both sides of the block.Alternatively, the porous block could be sprayed with a manganese oxidecoating, allowed to dry, and then be used.

The CPP must be sufficiently porous to allow migration of watertherethrough, but retain sufficient strength to withstand vehicle wheelloads typically encountered by road-side shoulders. One measure of theability of CPP to allow the migration of water is saturated hydraulicconductivity (K_(sat)) measured in cm/sec. For purposes of the presentinvention, the hydraulic conductivity of the CPP could range between 1.0and 0.001 cm/sec. One preferred embodiment has a hydraulic conductivityof about 0.01. While there may be situations where a very high hydraulicconductivity is desirable, this must be balance against concerns withsufficient structural strength and sufficient surface contact betweenthe pavement and the fluid flowing through it to insure mass transferand/or filtration by the pavement. The factors affecting the porosity ofthe CPP are the water to cement ratio, whether and how much pressure isapplied during curing, and to a lesser degree, the amount of fineaggregate in the mix.

A) Production of CPP as a Precast Uncoated Block

While there are many mixtures which would form the CPP of the presentinvention, three preferred mixtures are disclosed below in Table 1. Thewater cement ratio for each mix design is varied, ranging from 0.14 to0.32. However, these water cement ratios were used in conjunction withsteam curing as described below. Those skilled in the art will recognizethat if steam curing is not used, the chosen water cement ratio wouldprobably be higher. Nevertheless, to maintain a hydraulic conductivityof between 1.0 and 0.001 cm/sec., it is suggested that the water cementratio be maintained below 1. When CPP is used as a cast-in-placematerial (i.e. not steam cured) a water cement ratio of 0.3 to 0.4 wouldbe a recommended range.

Typically, the ratio of fine to course aggregates will be approximately1 to 1. While this ratio could vary, an excessive amount of fines maytend to reduce porosity by filling passages in the cement structure. Asan illustrative sample, the grain size distribution of the pea graveland sand used in Batch 2 is presented in FIG. 3.

TABLE 2 Mix Designs for Porous Pavement Block Error! Bookmark notdefined. Component Batch 1 Batch 2 Batch 3 Cement 109 kg 109 kg 109 kg(Type II) (240 lbs) (240 lbs) (240 lbs) Water 15-20 kg 20-25 kg 30-35 kgCoarse 472 kg 381 kg 431 kg Sand (1040 lbs) (840 lbs) (950 lbs) #9Gravel 336 kg — — (740 lbs) Pea — 381 kg 331 kg (840 lbs) (730 lbs) —Indicates material not used in batch

The pavement resulting from the disclosed mixes was formed in varioussizes of pre-cast blocks, for example 24 inches×16 inches×4 inchesthick. Naturally, this should by no means be considered an optimal size,but rather dimensioning of the blocks will depend on the application.The blocks were subject to a conventional pressurized, steam curingprocess. The process incorporates a press using hydraulic compression topress the concrete mix into the block form. The hydraulic press wascapable of exerting up to 35 kN (4 tons) of force on the wetcementitious mix in the form and the full 4 tons was applied in thisexperiment. Typically this pressure was applied for 1 to 4 minutes. Thenthe precast CPP blocks were steam cured (in a kiln with over 90%humidity) for four days to a week to promote adequate cement hydrationand then the blocks were allowed to air dry for two days beforetransport. Longer steam curing up to 28 days will produce a higherstrength material. Of course, there is a substantial amount offlexibility in the application of these various components in makingCPP. For example, although the experiment above used 4 tons of force (orabout 3000-lb/ft²) applied for approximately 1-minute, both the forceand duration of the loading can vary based on the application. Thoseskilled in the art will recognize many applications that may requireless force or applications requiring more or less duration of theloading.

From each porous pavement mix design, a block was sampled at random todetermine the strength and infiltration capacity. From each block fivecores are drilled using a 8 cm (3 in.) outside diameter diamond tippedcoring bit. This yielded cores approximately 7 cm (2.75 in.) indiameter. The infiltration capacity of the porous pavement blocks wasevaluated by the falling head permeability test for soils. Each core waswrapped with an impermeable membrane to determine hydraulic conductivityof the block. Flow was introduced from the bottom of the sample toensure complete saturation. Two trials were taken for each coreresulting in ten hydraulic conductivity values for each porous pavementmix design. As shown in Table 3, Batch 2 has the greatest hydraulicconductivity. Blocks tested later as full blocks had a full blockK_(sat) of approximately 0.01 cm/s.

Since the CPP on the roadway shoulder may be subject to occasionaltraffic loads (or many wheel loads in the case of parking areas), blockstrength is an essential consideration in the design. The unconfinedcompression strength of the blocks was evaluated. Two of the five coresfrom each mix design were tested to determine the unconfined compressionstrength. Since the length to diameter ratio of the cores was less than1.8, the strength was reduced by applying the appropriate correctionfactor as designated in ASTM C-39. The resulting compression strengthsof the three batches are seen in table 3

TABLE 3 Properties of the CPP Blocks Mix Average Hydraulic AverageUnconfined Design Unit Weight Conductivity (cm/sec) Compressive StrengthBatch 1 14.8 kN/m³ 0.0091 37,500 kPa (93.9 pcf) (5440 psi) Batch 2 14.1kN/m³ 0.0098 27,700 kPa (89.6 pcf) (4020 psi) Batch 3 14.6 kN/m³ 0.009033,600 (93.0 pcf) (4880 psi)

It is noted that these are only a few examples of measured properties ofCPP blocks. In other blocks, it is envisioned using CPP where thehydraulic conductivity values are designed either higher or lower thanthe above values by adjusting the water to cement ratio or adjusting thefine to course aggregate ratio.

B) Making Concrete Media, Cement Media or CPP with an AmphotericAdmixture

Previously described was a process of creating CPP blocks and thencoating the blocks with an amphoteric compound by soaking the blocks ina solution containing the amphoteric compound. However, the amphotericcompound could also be incorporated in the CPP as part of the process ofmixing the cement/aggregate slurry. An example of this method follows.

In a shallow container of large surface area compared to depth (in thelab environment, shallow Pyrex trays in the range of 12×16 inches wereused), there is placed a solution of 0.3 to 1.0 molar solution ofmanganese. The solution can be made by either method described above. Tothis solution, add a total of 1-kg of cement, and aggregate at thewater/cement ratio and cement/aggregate ratio of choice to produceconcrete of the strength and porosity desired. Those skilled in the artwill understand that whatever volume of amphoteric solution is addedshould count toward the total water cement ratio. For example, using 1kg of cement and a water cement ratio of 0.5, the adding of 0.25 kg ofamphoteric solution will require an additional 0.25 kg of water to beadded. The mixture is then dried (i.e. the cement is hydrated and theconcrete mixture hardens) approximately 12 hours. It should be notedthat at least part of the water in water-cement slurry is actually thesolution of manganese oxide. In effect, the entire cementitious materialis coated inside and out side with a manganese coating. The same methodcould be carried out for an iron oxide coating but with the onedifference; the CPP or cementitious media must be dried at an elevatedtemperature of 90 to 100 C. for at least 24 hours. As with all mediadiscussed above, if an iron oxide coating is not fully dry beforerinsing, some of the coating will be washed off. This typically is not aconcern with manganese oxide coatings since manganese oxides usuallybond far better to substrates such as CPP (and polymer beads) than ironoxides.

With cementitious material as a porous matrix (i.e. as a substrate),final pH conditioning of the iron oxide coating is not required becausethe alkaline nature of the cement raises the pH to acceptable levels. Infact, the acidic nature of the iron oxide solution (and to a lesserextent the manganese oxide solution) actually creates more internalporosity of the CPP by consuming a portion of the cement matrix througha neutralization reaction. However, this increased internal porosityalso results in a reduction in the cement matrix's strength. This isproblem which is much less prevalent when manganese oxide is theamphoteric compound.

One useful application of a CPP coated with an amphoteric compound is asa roadway runoff filtering shoulder. FIG. 4 illustrates a cross-sectionof a typical roadway. The roadway will have driving lanes 5 withshoulders 4. In FIG. 4, the shoulders are formed of a CPP having anamphoteric compound coating as described above. Typically, the CPPshoulder will have a thickness ranging from 4 to 16 inches. Rainwaterrun-off depicted by arrows 6 will flow off of the driving lanes 5 andonto the CPP shoulders 4. The runoff will percolate into the CPPmaterial and metal ions will be sorbed by the amphoteric compound on theCPP material. The runoff (with metal ions removed) will then flow out ofthe side and bottom of the CPP shoulders 4.

Another application of concrete produce with an amphoteric solution isuse as a crushed aggregate filter media. In other words, the object isnot to have water flow through the individual pieces of concrete, but tohave it flow around broken up concrete rubble. To create a concretemedia or cement media that is fully impregnated with manganese, thewater-cement ratio would be higher to ensure sufficient cohesive andadhesive bonding within each piece of media. In this situation, thewater cement ratios are close to that of standard concrete mixes and apreferred range would be 0.40 to 0.90. This water cement ratio includesthe aqueous solution gained from the admixture. This will be referred toas the “aqueous solution cement ratio” to imply that both water and theadmixture solution are considered in computing the ratio. The concretewould be mixed as above and once it hardens (from example, after 12hours), it is broken up as rubble into media sizes of choice. Typicallythese sizes can range from 0.2 to 10 mm. The rubble could then be coatedwith an amphoteric compound such as described above in regards topolyethylene beads.

Another preferred method of coating the CPP (or other substrates)includes recoating the media. One example of recoating the media wasaccomplished by placing the media in a column in which it will befluidized with a recirculating flow of manganese solution. Thus, 1-kg ofmedia was placed in a vertical column (the column was approximately 2liters in volume) with a 6-liter recirculating solution of 10⁻³ M NaHCO₃and 0.035-moles/liter Mn²⁺ (stoichiometric amount) and re-circulatingthis solution with a pump capable of handling aggressive solutions andwith a sufficient capacity to fluidize the bed. The Mn²⁺ is oxidized byadding 250-mL of a 0.185 M solution of NaOCl at a flow rate of 5mL/minute for 1 hour to ensure complete oxidation of the manganese. Themanganese oxide in this solution is then re-circulated for an additional2 hours with 250-mL of 0.185 M NaOCl added in one step at the beginningof the 2 hours. After 2 hours, the solution was drained and thenreplaced with water (in the lab, it was de-ionized (DI) water) andre-circulated for 15 minutes and then the column was drained of thewater solution. The media was then rinsed with water (DI in lab) to a pHof 7 and then allowed to dry overnight before use. The rising of amanganese oxide coated media with DI water was mainly to removeimpurities in order to obtain laboratory quality samples. In practicalfield applications, the final rinsing of manganese oxide coated mediacould be dispensed with.

Naturally, re-coating of the media is not limited to manganese oxideupon manganese oxide. Another re-coating method would include a firstcoating with iron oxide followed by a second coating of manganese oxide.If the iron oxide coated material produces a sufficiently high SSAsubstrate for the intended application, this latter method may be moredesirable since iron oxide is normally less costly than manganese oxide.Thus, a comparatively inexpensive substrate such as sand with a low SSAmay be coated with iron oxide to produce a comparatively high SSAsubstrate (i.e. a substrate with a SSA much greater than 0.1 m²/g). Inother words, the iron oxide coated sand becomes the substrate for thefinal filter media which is coated with manganese oxide. Additionally,the increased SSA achieved by re-coating may be applied to any of theabove disclosed substrates (CCP, wood, polymers, etc.) or with otheroxides such as aluminum or other surface active materials of highsurface area and amphoteric nature.

Additionally, substates could be formed from any porous structure havinga fixed matrix. An example of such a porous fixed matrix would besolidified lava (or lava rock). A fixed matrix having a porosity ofbetween 0.05 and 0.6 would be suitable for use in the present invention.

6. Other Embodiments of the Present Invention

The present invention may be put to enumerable uses. For example, whilethe above disclosure discusses a cementitious porous pavement material,the porous pavement material could also be bituminous or asphaltic.Porous asphalt can be made by reducing the asphaltic binder and, ineffect, producing a lower binder—aggregate ratio. Typically, theamphoteric compounds described above may also be added to the bituminousporous pavements during the mixing stage, creating the same type ofwaterborne metals filter. However, with all porous materials, anamphoteric material can always be added as a surface coating and much ofthe porous surface can be coated by application of a spray on the poroussurface.

Large areas of porous pavements may also be used as storm water storagebasins. Parking lots and similar large paved areas are often the sourceof significant volumes of storm water runoff The porous pavement of thepresent invention provides a means of substantially reducing the volumeof runoff from such large pavement areas. These areas may be defined asa ratio of their length to width. For purposes of the present invention,a storage basin may be any pavement area having a length to width ratio(i.e. length/width) of less than 20. FIG. 5 is an illustrative exampleof a storage basin 20 which takes the form of a parking area 21 havingparking spaces 22. Storage basin 20 has a length “l”, a width “w”and agreatly exaggerated depth “d”. A typical parking area formed of porouspavement would have a porous pavement with a hydraulic conductivity ofbetween 0.0001 cm/sec and 1.0 cm/sec and more preferably of around atleast 0.001 cm/sec. Because it is not necessary to transfer the water soquickly in parking areas, it preferred to have higher strength and lowerporosity. Porous pavement having a hydraulic conductivity of 0.001cm/sec. and 1.0 cm/sec will normally have a strength of betweenapproximately 3000 psi and 5000 psi. The void volume will beapproximately 20% to 30% of the total volume of the concrete. Such alayer of porous pavement should be at least six to eight inches in depthand preferably, at least twelve to fifteen inches in depth. This depthprovides both the necessary strength to support vehicular traffic andalso provides a sufficient volume of pore space to store the water froman average rain storm. With a 20% to 30% pore volume, a 6 inch slab ofporous pavement could retain as much as 1 to 1.8 inches of rainfall uponthat slab. Rather than placing further strain on storm sewers, the raincollected in the porous pavement will be left to evaporate during dryerdays. This method of storing runoff from parking lots has the furtherbenefit of tending to immobilize parking lot pollutants entrained by therainwater. Rather than leaving the premises of the parking lot, suchpollutants will be retained in the porous pavement. As the waterevaporates from the porous pavement over time, the pollutants will tendto be retained in the pavement. Many pollutants may be volatize into theair during evaporation, a process which is preferable to the pollutantsbecoming dissolved in water. Additionally, the porous pavement may betreated with an amphoteric compound in order to improve the capture ofwaterborne ionic constituents which are held in the porous pavementwhile the retained water evaporates. It can readily be seen how aparking lot constructed of porous pavement will form a storm waterstorage basin capable of supporting vehicular traffic.

Another embodiment of the present invention includes a roadway gravelshoulder capable of capturing waterborne ionic constituents entrained inroadway rain runoff. Roadways often have gravel shoulders at least fourinches in depth, more typically six to eight inches in depth and forlarger roadways, often over eight inches in depth. Commonly, the gravelfor roadways is graded to have an average diameter of betweenthree-fourths of an inch to one inch. To carry out this aspect of theinvention, the gravel may be coated with an amphoteric compound such asone of the iron oxides or manganese oxides disclosed above. Preferably,this would be done prior to placing the gravel as a roadway shoulder.Any of the coating processes discuss above would be suitable, but thepreviously described field method for producing large quantities of ironoxide coated sand would be one preferred method. The gravel could alsobe subject to the multiple layer coating also described above. Once thecoating process for the gravel was complete, the gravel would be placedalong the roadside in the normal manner for creating a shoulder. Thismanner of capturing waterborne ionic constituents is advantageousbecause it can passively filter and treat pavement sheet flow directlyat the edge of the pavement before the flow becomes concentrated.

A still further embodiment of the present invention encompasses coatinga flexible, planar, porous substrate with an amphoteric compound. Oneexample of a flexible planar, porous substrate would be geosyntheticfabrics which are well known in the art. Geosynthetic fabrics aregenerally polymeric materials which are designed to be placed in oragainst soil. Often geosynthetic fabrics are used to retain soil inplace while allowing water to pass through the fabric. Geosyntheticfabrics may be woven or nonwoven. Woven geosynthetic fabrics are fabricswith filaments in warp (machine direction) and weft (cross-machine)direction. Nonwoven fabrics have essentially a random fabric or textilestructure. For example, common felt is a nonwoven textile. Nonwovens arefurther characterized according to how fibers are interlocked or bonded,which is achieved by mechanical, chemical, thermal or solvent means.Some of the polymeric materials used to construct geosynthetic fabricsinclude: polyethylenes—PE, HDPE, LDPE, XLPE, FLPE, CPE, CSPE;polypropylene—PP, polysulfone—PSF; polyurethane—PUR; polycarbonate—PC;polyvinyl chloride—PVC, polystyrene—PS; thermoplastic elastomer—TPE;nylon—PA; polyester—PET; nytrile; butyl; acetal—ACL; and polyamide—PA.Most typically, geosynthetics are formed from PE, PP, PVC, PET, PA orPS. The application of an amphoteric coating to the geosynthetics couldbe carried out by a process similar to that described above for coatingpolyethylene beads. However, rather than stirring the beads, the sheetsof fabric are dipped in solution, pulled them out of the oxide solution,and then dried them. The sheet could be left in the solution whiledried, but this method wastes a substantial amount of oxide solution.With fabric or sheet material, the preferred techniques will be to sprayon the solution and dry or a dip in the solution and dry.

Geosynthetic materials coated with amphoteric oxides can serve as moreeffective filters (higher surface area and surface roughness) which canadsorb cations (e.g. heavy metals) or anions (e.g. phosphates) dependingon the pH of the aqueous stream, seepage, ground water, or the like. Thefilters of the present invention can be in-situ or ex-situ. An exampleof an in-situ filter would be where one has shallow contaminatedgroundwater or one is directing a flow of storm water into a trench. Onecan place a sheet of oxide coated geosynthetic in a trench, backfillaround it and let the flow passively move through the trench andtherefore move through the more permeable geosynthetic to providein-situ treatment. Ex-situ filters would be all of those cases where onedoes treatment in some form of a device or reactor, like the upflowcolumn seen in FIG. 1.

Another example of a flexible planar, porous substrate would be membranematerials. Membrane materials typically have much smaller pore sizesthan other filters, commercially available on the order of 0.1 to 50microns and can be up to 3000 or more microns. Often membrane materialsare formed from a type of cellulose such as cellulose acetate, celluloseesters, cellulose nitrate, or nitrocellulose. The amphoteric coating maybe applied as described above for oxide coated geosynthetics. Themembrane substrates may be considered “membrane filters” in the sensethat they capture constituents only on their surface. This isdistinguished from the other substrates described herein which act as“depth filters.” Depth filters capture constituents through some depth(even if relatively shallow) in the substrate.

The flexible planar, porous substrate could also include any number ofconvention filter materials or devices which have a larger areadimension than depth dimension. For example, conventional airconditioning or furnace cartridge filters could be formed by having anamphoteric compound applied to the filter media within the cartridge.The filter media will typically be a fiberous polymeric or glassmaterial woven or meshed together at different densities depending onthe intended use of the filter.

A further embodiment of the present invention includes a drainage pipecapable of capturing waterborne ionic constituents. Most storm waterrunoff is carried through conventional concrete pipes for at least partof the journey to its final collection point. Thus there is theopportunity to bring the runoff into contact with a pipe surface coatedwith an amphoteric compound and remove ionic constituents from thewater. Typically, drainage lines are sized to accommodate a standardrunoff rate which is less than the total capacity of the drainage pipes.In other words, drainage lines are not designed to have the averagerunoff completely fill the volume of the drainage pipe. This means thatless than the entire inner circumference of the pipe is designed to comeinto contact with the runoff water. Therefore, it may not be necessaryto coat the entire interior of the pipe with the amphoteric compound,but rather only coat the portion of the inner pipe surface designed tobe in contact with the water. It will be obvious that the decisionconcerning how much of the inner surface of the pipe should be coated isa engineering design choice which will vary according to the designparameters. One manner of applying the amphoteric compound will simplybe to immerse the section of pipe to be coated in an amphoteric compoundcontaining solution such as disclosed above. For example, the solutioncould be a 1 to 3 molar ferric nitrate or ferric chloride solution or a0.5 to 2 molar solution of either birnessite or cryptomelane.Alternatively, the amphoteric solution could be applied directly to thepipe surface by spraying and the like.

The piping could be formed out of conventional concrete or a CPPmaterial such as described above. The CPP piping would most likely beused when the pipe grade was above the water table or placed in soilwhich could otherwise readily absorb runoff. In this manner, runoffflowing through the water could be at least partially returned to theground around the run of the pipeline. The CPP piping would typicallyhave a hydraulic conductivity ranging from about 0.001 to about 1.0cm/sec. Both the CPP piping and conventional concrete piping could havethe amphoteric compound introduced in the mixing process prior to theconcrete mixture being placed in the pipe forms. It is also in the scopeof the present invention to include conventional fired clay piping whichhas been coated with an amphoteric compound or a specially made claypiping which has had the amphoteric compound added as part of the claymixture before the pipe is fired.

Another embodiment of the present invention comprises forming a filterby placing an amphoteric compound in a clay liner or in a roadwaysub-base. As used herein, the term “sub-base” is intended to include aroadway sub-base formed of clay, silt or sand or a mixture of thesematerials or recycled materials. This sub-base may be water pervious orimpervious. Conventionally, a sub-base is formed by placing a layer ofuncompacted soil or recycled material over the area where the sub-baseis to be constructed. Water is then added to bring the sub-base to itsoptimum compacted moisture content. The layer is then compacted to apredetermined density. Typically, this process is carried out in layersor “lifts” as is well known in the art. The optimum compacted moisturecontent is determined by standard testing procedures such as set out inASTM D698. An improved sub-base according to the present invention maybe constructed by raising the uncompacted sub-base to its optimumcompacted moisture content with a solution containing an amphotericcompound. It may not be necessary to add the amphoteric solution to alllifts, but simply the upper most 1 to 3 lifts. Clays have a wide rangeof SSA values ranging from approximately 15 m²/g for clays likekaolinite or illite up to approximately 850 m²/g for clays like sodiummontmorrilite. Their large SSA values make clays a highly effectivesubstrate for applying amphoteric compounds.

Another geotechnical structure utilizing amphoteric compounds could bewater impervious clay liners. While clay liners are intended to be waterimpermeable, it is common for liners to have some permeability resultingin water escaping from within the liner into the surrounding soil. Ifthe clay liner is treated with an amphoteric compound, water travelingalong the liner (toward the break) or through the liner will have ionicconstituents sorbed from it. In a similar manner, some roadways arebuilt with sub-bases which are intended to be water impervious.Generally, it is also not intended to have water flow through thepavement to the sub-base. However, cracking in roadways is commonplaceand rainwater migrates through the cracks to the sub-base. If thesub-base retains its water impermeable characteristics, water will flowlaterally to the edge of the roadway. If the sub-base is coated with anamphoteric compound, ionic constituents are effectively removed as thewater travels along the sub-base toward the edge of the roadway. If thesub-base also forms cracks, water flowing through the sub-base will betreated.

While the foregoing invention has often been described in terms ofspecific examples, those skilled in the art will recognize manyvariations which are intended to fall within the scope of the claims.For example, while manganese and iron are two preferred elements forfrom amphoteric compounds, aluminum is a third element which may beutilized. Furthermore, while the above has described the media asutilized for remove heavy metals from water, the media could be utilizedto remove many types of airborne or waterborne ionic constituents. Inparticular, sand or polyethylene beads filters could readily be adaptedto treat flows of air for ionic constituents such as aerosols, chargedparticulate matter, odors, and gas emissions containing water vapor withanionic or cationic species. All of these variations are intended tocome within the scope of the following claim.

What is claimed is:
 1. A pavement material for the capture of waterborneconstituents, said material comprising: a hydraulically conductiveporous pavement substrate having a surface and a depth; and anamphoteric compound bonded to said substrate though at least part ofsaid depth such that water contacting said surface may pass through saidsurface and come into contact with said amphoteric compound as saidwater passes through said porous pavement.
 2. The pavement materialaccording to claim 1, wherein said porous pavement substrate is acementitious pavement.
 3. The pavement material according to claim 1,wherein said porous pavement substrate is an asphalt pavement.
 4. Thepavement material according to claim 1, wherein said substrate has ahydraulic conductivity ranging from about 0.001 to about 1.0 cm/sec. 5.The pavement material according to claim 4, wherein said substrate has ahydraulic conductivity ranging from about 0.001 to about 1.0 cm/sec. 6.A pavement material according to claim 1, wherein said amphotericcompound is an oxide of the group consisting of iron, manganese, andaluminum.
 7. A pavement material according to claim 2, wherein saidamphoteric compound is an oxide of the group consisting of iron,manganese, and aluminum.
 8. A pavement material according to claim 3,wherein said amphoteric compound is an oxide of the group consisting ofiron, manganese, and aluminum.
 9. A pavement material according to claim1, wherein said amphoteric compound is bonded to said porous pavementthrough substantially all of said depth.
 10. A pavement materialaccording to claim 1, where said amphoteric compound is bonded onto saidporous pavement as a coating of said amphoteric compound.
 11. A pavementmaterial for the capture of waterborne constituents, said materialcomprising: a. a hydraulically conductive porous pavement substratehaving a surface and a depth; and b. an inorganic amphoteric compoundbonded to said substrate though at least part of said depth such thatwater contacting said surface may pass through said surface and comeinto contact with said amphoteric compound as said water passes throughsaid porous pavement.
 12. A pavement material according to claim 11,wherein said amphoteric compound is an oxide of the group consisting ofiron, manganese, and aluminum.